Reptile Fossil Skull Reveals How Snakes Evolved Without Limbs

Modern snake skull, with inner ear shown in orange. The reconstructed 3D models revealed a distinctive structure suggesting the animals evolved without limbs when their ancestors began burrowing. (Photo : Hongyu Yi)

A recently unearthed 90 million-year-old fossilized reptile skull is shedding new light on how snakes lost their limbs, according to researchers from the University of Edinburgh.

"How snakes lost their legs has long been a mystery to scientists, but it seems that this happened when their ancestors became adept at burrowing," Dr. Hongyu Yi, leader of the study from the University's School of GeoSciences, explained in a news release. "The inner ears of fossils can reveal a remarkable amount of information, and are very useful when the exterior of fossils are too damaged or fragile to examine."

Previously, scientists believed snakes lost their limbs as a means for living in the sea. However, the recent study compared CT scans of modern and fossil reptiles and found that the slithering animals lost their limbs when their ancient ancestors began living and hunting in burrows instead. This evolutionary adaptation is still present in snakes today.

Using the CT scans, researchers were able to specifically examine the bony inner ear of Dinilysia patagonica, which is an ancient two-meter-long relative of modern snakes. These interior bony canals and cavities of modern burrowing snakes ultimately control hearing and balance.

Knowing this, researchers constructed 3D virtual models to compare fossil and modern inner ears of lizards and snakes. In doing so, they found a distinctive structure within the inner ear of actively burrowing animals that was not found in modern snakes that live in water or above ground. This evolutionary characteristic may help the animals detect prey and predators, researchers noted.

Essentially, the recent findings help scientists fill in the gaps of snakes' mysterious and limbless evolution. It also confirmed Dinilysia patagonica was the largest burrowing snake ever known.

"This discovery would not have been possible a decade ago -- CT scanning has revolutionized how we can study ancient animals. We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles," Mark Norell, one of the study researchers from the American Museum of Natural History, who took part in the study, concluded in the university's release.